SUMMARY/ABSTRACT Venous thromboembolism (VTE) is the third leading cause of death from cardiovascular disease behind heart attack and stroke. Annually, VTE affects approximately 900,000 Americans with as many as 30% of VTE patients dying within 30 days. VTE begins as a deep vein thrombosis (DVT), or blood clot in a deep vein. Left untreated, DVT can break free from the vein wall, travel to the lungs and block some or all of the blood supply as a pulmonary embolism (PE). Much of the morbidity and mortality associated with VTE could be prevented with the early, rapid and accurate diagnosis of DVT, allowing for early intervention and decreasing the risk of fatal PE. However, diagnosing DVT/VTE is challenging, resulting in both underdiagnosis and overtreatment with anticoagulants. DVT patients present with variable and non-specific symptoms, and ~50% people with DVT are asymptomatic. Sudden death is actually the first symptom in ~25% of people with PE. Upon clinical assessment, patients suspected of having DVT/VTE are subjected to Wells criteria scoring and D-dimer blood testing, which are only valuable as rule out tests. Despite availability of recommended pretests, only 20% of patients evaluated for DVT and ~5% evaluated for PE via diagnostic imaging actually have the disease. Further, 33% of patients screened for VTE are unnecessarily treated with anticoagulants, which is associated with a high mortality risk due to bleeding. While advanced imaging can diagnose DVT/VTE, it is expensive, time-consuming, harmful to some patients, not always accurate, and is often not available in the outpatient setting where 73% of VTEs occur. Thus, there is an urgent unmet need for a rapid DVT/VTE diagnostic that can be performed at the point of care. The pathobiology of DVT/VTE has been characterized and several plasma biomarkers have shown promise in their ability to confirm DVT, though individually, these biomarkers lack the required sensitivity and selectivity for reliable DVT detection in patient studies. Identification of a novel panel of peptides that recognize multiple serological markers (i.e. seromarkers) of DVT simultaneously has the potential to overcome these limitations and yield a robust diagnostic for DVT/VTE. We propose to identify a sensitive and selective peptide signature for DVT, which we will develop into a reliable, cost-effective, non-invasive, diagnostic assay that can be performed at the point of care to improve DVT/VTE patient outcomes and reduce unnecessary medical costs. In this Phase 1 application, we will identify peptides that bind seromarkers uniquely associated with acute DVT patients. We will then validate a subset of these peptides that, collectively, can accurately and reproducibly detect DVT from patient plasma. In Phase 2, we will develop this peptide signature into a novel assay capable of objectively diagnosing DVT/VTE rapidly at the point of care, and scale up assay production.